Multi-user multiple-input multiple-output MU-MIMO) technologies are widely applied to both conventional non line of sight (NLOS) outdoor macro base station scenarios in which there are relatively many obstructions and reflections, and direct line of sight (LOS) indoor scenarios (for example, distributed MIMO) in which there are relatively few obstructions and reflections.
In an MU-MIMO system, when multiple users transmit data on a same time-frequency resource, each user not only receives data streams sent from a base station to the user, but also receives interference signals from other users. An effective solution to effectively suppress interference between users and improve system capacity and spectral efficiency is a beamforming (BF) technology, that is, a base station side performs weighting by using a pre-designed beamforming vector before transmitting data, so that a main lobe of an antenna directivity pattern is aligned with a user direction and a null point is aligned with an interference direction when the base station sends data of the user, thereby improving a signal-to-noise ratio of a direction of arrival of user equipment (UE) and achieving the objective of interference suppression.
However, to design a beamforming vector in null space of an interference user, the base station side needs to know downlink channel information of the user.
In a frequency division duplex (FDD) system in which an uplink and a downlink are asymmetric, a base station cannot obtain downlink channel information by using an uplink channel.
In a time division duplex (TDD) system in which only one antenna participates in uplink transmission and two antennas participate in downlink reception, due to reciprocity between an uplink channel and a downlink channel, a base station can obtain half of downlink channel information by measuring the uplink channel, but cannot obtain complete downlink channel information from the TDD system.